UV‐B increases the harvest index of bean (Phaseolus vulgaris L.)

The effects of small changes in natural UV‐B on the photosynthesis, pigmentation, flowering and yield of bean plants (Phaseolus vulgaris L. var. Label) were studied. To obtain a relatively natural growth environment, the plants were grown in small, half‐open greenhouses of UV‐transmitting Plexiglas of different thickness (3 and 5 mm), resulting in an 8% difference in the weighted UV‐B reaching the plants. Although the UV‐B doses used did not significantly influence photosynthesis on a leaf area basis during vegetative growth, important changes in biomass allocation were noted. A UV‐B‐O induced reduction in leaf area during the period of vegetative growth resulted in decreased dry weight after 57 d. During the flowering and pod‐filling stages (57–79.d after planting), however, plants grown at high UV‐B retained their photosynthetic capacity longer: maximal photosynthesis, chlorophyll and N content of the leaves were higher under the higher UV‐B dose at a plant age of 79 d. Combined with an increased allocation under the higher UV‐B dose of both N and biomass to the pods, this resulted in a small increase in yield and an important increase in harvest index with increased UV‐B.